The invention relates to a method for ascertaining a correction value for the angular position of the rotor of an electrically commutated reversible synchronous motor on the basis of a sensor used for driving the synchronous motor.
A sensor may be used with a permanent magnet synchronous motor in an elevator door control system. Mechanical phase displacement angles can occur during fitting of the sensor or as a result of hystereses. The error can cause inaccuracies and/or errors and result in an erroneous driving of the synchronous motor.
When driving electrically commutated synchronous motors, the exact knowledge of the rotor position in relation to the winding layer of the stator is absolutely necessary for optimum development of force with minimum noise. With a view to minimum development of noise, the motors are generally operated with sinusoidal driving and, for example, resolvers, optical and magnetic absolute value transmitters (photoelectric barriers and Hall elements) or incremental sensors with one or more indexers are used as sensors.
The position between the motor winding and the sensor is predetermined mechanically; the relative position to one another can vary as a result of fitting inaccuracies, however. A further source of fluctuating angles are hysteresis effects of the sensors used. These variations necessarily result in erroneous driving of the motor since the measured angular position and the actual angular position do not exactly correspond to one another. In particular in the case of multipolar motors, this error has a very negative effect since a mechanical fitting error with the factor of the pole pair number as an electrical phase displacement angle becomes noticeable during the driving. For example, a fitting inaccuracy of 1° in the case of a motor with ten pole pairs brings about an electrical phase displacement angle of 10°. Based on the leading angle of 90°, this already corresponds to erroneous driving of 11%. This results in a reduction in the effective torque and in an increase in the development of noise and in increased losses.
Until now, the electrical phase displacement angle has been reduced by mechanical sensor compensation or by interactive or automated (for example during manufacture) inputting of a compensation value for the angular offset into the control electronics. There are even sensors on the market which are already internally prepared for processing such an offset value. In the sector of door controllers, electrically commutated motors are still seldom used and automatic compensation methods have not yet previously been used for these motors.